1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to systems and methods of providing discrete Raman amplification of optical signals.
2. Statement of the Problem
Many communication companies use fiber optic cabling as a media for transmitting data because of its high-bandwidth capacity. Fiber optic cables reliably transport optical signals over long distances. Over a distance, optical signals attenuate in the fiber due to Rayleigh scattering. The attenuation may be recovered by an optical amplifier.
Optical amplifiers may be discrete amplifiers or distributed amplifiers. A distributed amplifier uses the transmission fiber carrying the optical signals as a gain medium. A discrete amplifier does not use the actual transmission fiber as the gain medium, but is a separate component that includes a span of rare-earth doped fiber, Dispersion Compensating Fiber (DCF), Highly Nonlinear Fiber (HNF), or another type of fiber as the gain medium.
One type of discrete amplifier is an Erbium-Doped Fiber Amplifier (EDFA). An EDFA comprises a span of Erbium-doped fiber and a Raman laser diode. The Erbium-doped fiber receives optical signals to be amplified. The laser diode transmits a laser having a wavelength of 980 nm onto the Erbium-doped fiber concurrently as the optical signals travel over the Erbium-doped fiber. The properties of the Erbium-doped fiber act to absorb the pumped laser and generate a gain in the optical signals using the absorbed laser. EDFAs are generally used to amplify optical signals in the C-band (1530 nm to 1560 nm). Other types of rare earth-doped discrete amplifiers are used to amplify other bands, such as the S-band and the L-band. Rare earth-doped discrete amplifiers usually have a fixed gain range that depends on the dopant used for the gain medium.
Another type of discrete amplifier is a discrete Raman amplifier. A discrete Raman amplifier comprises a span of fiber and a Raman pump. The fiber may be Dispersion Compensating Fiber (DCF) or Highly Nonlinear Fiber (HNF). Conventional discrete Raman amplifiers use a length of fiber between 4 to 10 km. The fiber receives optical signals to be amplified. The Raman pump transmits a laser onto the fiber concurrently as the optical signals travel over the fiber. Due to the Raman effect, the laser generates a gain in the optical signals.
The gain range of discrete Raman amplifiers is flexible and depends on the wavelength of the pump laser. The laser amplifies wavelengths at one Raman Stokes shift from the laser wavelength. A first order Raman Stokes shift comprises the wavelengths about 100 nm longer than the pump laser wavelength in glass fiber. For instance, a 1455 nm pump laser wavelength amplifies optical signals having wavelengths around 1550 nm. The gain bandwidth is about 30 nm centered about the 1550 nm wavelength. This configuration is shown in FIG. 1 and discussed below.
One problem with current discrete Raman amplifiers is the high amount of double Rayleigh scattering occurring in the amplifier. The high amount of double Rayleigh scattering is due to the long length of fiber used as the gain medium. The amount of double Rayleigh scattering in the fiber is proportional to the length of the fiber. Double Rayleigh scattering limits the amount of gain that can be achieved in the discrete Raman amplifier.